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Time Conversion and Formats

Time Conversion and Formats

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Time Conversion and Formats

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  1. Time Conversion and Formats June 2004

  2. Overview • Kernels • Parsing Time Strings • Calendar, DOY, Julian Date • Spacecraft Clock • Converting Numeric Times • Creating Time Strings - Formats • Calendar, DOY, Julian Date • Spacecraft Clock • Local Solar Time • Customizing the Time System • Layout of the Time System Time Conversion and Formats

  3. Kernels • Three time systems are commonly used in SPICE for inputs and outputs: • Coordinated Universal Time (UTC) • Spacecraft Clock (SCLK) • Ephemeris Time (ET, also referred to as Barycentric Dynamical Time, TDB) • Two time systems are commonly used in SPICE for kernel data look-ups or for computations • Ephemeris Time (ET, TDB) • Used in SPK, frames, PCK • Used in high-level geometry interfaces • Ticks (encoded spacecraft clock) • Used in the intermediate level CK interfaces • The SPICE Leapseconds Kernel and Spacecraft Clock Kernel capture the near-term relationships between • UTC and ET (Leapseconds Kernel) • UTC or ET and SCLK (SCLK Kernel) • The long-term relationships between these systems cannot be accurately predicted Time Conversion and Formats

  4. Parsing Time Strings • UTC, TDB, or TDT (TT) strings to Ephemeris Time • STR2ET ( string, ET ) • String formats may be Calendar, DOY, Julian Date • Requires Leapseconds Kernel (LSK) • Spacecraft Clock to Ephemeris Time • SCS2E ( scid, string, ET ) • Requires Spacecraft Clock Kernel (SCLK) • Normally requires Leapseconds Kernel (LSK) as well • To handle a very small (~2 msec.) difference between TDB and TT • Spacecraft Clock to “Ticks” (used in the mid-level interfaces of the C-kernel system) • SCENCD (scid, string, ticks) • Requires Spacecraft Clock Kernel (SCLK) Time Conversion and Formats

  5. Converting Numeric Times • Between Spacecraft Clock "Ticks" and Ephemeris Time • SCT2E ( scid, ticks, ET ) • SCE2C ( scid, et, TICKS ) (continuous ticks) • SCE2T ( scid, et, TICKS ) (discrete ticks) • Requires Spacecraft Clock Kernel (SCLK) • Normally requires Leapseconds Kernel (LSK) as well • To handle a very small (~2 msec.) difference between TDB and TT • Between any pair of TDB, TDT (TT), TAI, JED, JDTDT represented as double precision numbers • RESULT = UNITIM ( epoch, insys, outsys ) • Requires Leapseconds Kernel (LSK) • Find ET-UTC ("DELTA ET", "DUT") at specified ET or UTC time • DELTET ( epoch, eptype, DELTA ) Time Conversion and Formats

  6. Creating Time Strings - Formats - 1 • Converting Ephemeris Time to Calendar, DOY, Julian Date String • TIMOUT • Requires Leapseconds Kernel • ET2UTC • Requires Leapseconds Kernel • ETCAL • Produces Ephemeris Calendar Time Strings (TDB). • Fixed format. • No Leapseconds Kernel is required. Time Conversion and Formats

  7. Creating Time Strings - Formats - 2 • TIMOUT ( et, fmtpic,STRING ) • etis the familiar seconds past J2000 • fmtpic (format picture) is a specification giving the user great flexibility in setting the appearance of the output time string • YYYY Mon DD, HR:MN:SC.### ::UTC • 1999 Jan 12, 23:28:28.289 • YYYY Mon DD, AP:MN:SC.### ampm ::UTC-8 (PST) • 1999 Jan 12, 03:28:28.289 p.m. (PST) • See the header for the TIMOUT module • The module TPICTR may be useful in constructing a format picture specification from a sample string Time Conversion and Formats

  8. Times 1999-283T12:28:29 (UTC) 1999-283T12:29:33 (TDB) Wed Nov 03, 19:29:05 1999 465 B.C. Jan 12 03:15:23 p.m. 1987-11-03T04:29:58 04:28:55 A.M. June 12, 1982 Thursday November 04, 1999 DEC 31, 15:59:60.12 1998 (PST) Format picture used YYYY-DOYTHR:MN:SC (UTC) YYYY-DOYTHR:MN:SC (TDB) ::TDB Wkd Mon DD, HR:MN:SC YYYY YYYY ERA Mon DD AP:MN:SC ampm YYYY-MM-DDTHR:MN:SC AP:MN:SC AMPM Month DD, YYYY Weekday Month DD, YYYY MON DD, HR:MN:SC YYYY (PST)::UTC-8 Creating Time Strings - Formats - 3 Time Conversion and Formats

  9. Creating Time Strings - Formats - 4 • Converting Ephemeris Time to Spacecraft Clock Strings • SCE2S • Requires both LSK and SCLK • Converting Ticks to Spacecraft Clock Strings • SCDECD • Requires SCLK • Converting Ephemeris Time to Local Solar Time Strings • ET2LST • Requires SPK, PCK Time Conversion and Formats

  10. Customizing the Time System • Defaults • Two digit year (a bad idea but supported):1969-2068 • Time System: UTC • Calendar: Gregorian • Adjustments • The one hundred year interval to which two digit years belong may be set. For example 1980-2079 • Time Systems: UTC, TDB, TT (Terrestrial Time) • Calendar: Gregorian, Julian, or Mixed. • See TIMDEF or Time Required Reading (time.req) for details Time Conversion and Formats

  11. Coordinated Universal Time (UTC) Local Solar Time UTC seconds past J2000 Uniform time systems (TDT,TAI, JED, JDTDT) Barycentric Dynamical Time (TDB or ET) TIMOUT ET2UTC SCE2S SCS2E SCE2C (SCE2T) DELTET STR2ET UNITIM SCT2E SCDECD Spacecraft Clock (SCLK) Encoded Spacecraft Clock (Ticks) SCENCD Layout of the Time System needs lsk needs sclk needs lsk and sclk needs pck and spk ET2LST Time Conversion and Formats